But in 1977, Mr. Carter and his attorney general, Griffin B. Bell, authorized warrantless electronic surveillance used in the conviction of two men for spying on behalf of Vietnam.

The men, Truong Dinh Hung and Ronald Louis Humphrey, challenged their espionage convictions to the U.S. Court of Appeals for the 4th Circuit, which unanimously ruled that the warrantless searches did not violate the men's rights.

In its opinion, the court said the executive branch has the "inherent authority" to wiretap enemies such as terror plotters and is excused from obtaining warrants when surveillance is "conducted 'primarily' for foreign intelligence reasons."